Crystalline wafers are usually produced by sawing crystalline rods into slices. A problem arises near the end of the separation operation when there is only a thin, increasingly mechanically fragile residual connection between the wafer and rod. The forces which are produced on the wafer during the separation operation finally twist or break off the residual connection, leaving a stump on the surface of the wafer, which has to be removed in a further working step. Additionally, breaks often occur in the surface of the wafer, which render the wafer of no further use.
In order to avoid the problems which occur near the end of the separation operation, the wafer should be supported as securely as possible. Experience has shown that the wafer cannot be satisfactorily supported by means of grippers or the like, since scratches on the wafer, or even breaks in its edges, occur at the points of contact with the gripping devices. Furthermore, the crystalline wafers, which are usually very thin, bend when gripped too securely, with the result that a wafer having parallel faces can no longer be produced.
A holding tool, which works on aerodynamic principles, has been developed. It has a circular holding face, in the center of which is a counter-sunk nozzle from which gas flows at an acute angle to the holding face. At a certain spacing of the holding face from the wafer, the wafer is forced toward the holding face by a reduction in pressure between the wafer and the holding face caused by the gas flowing between the wafer and the holding face according to the Bernoulli principle. In order to prevent the wafer from sliding or wobbling, the holding tool has a raised stop extending around two thirds of the holding face. The gas flow causes the wafer to contact the stop and scratches or breaks can be produced on the wafer. Such damage can occur as the wafer is released, if it slips sideways or slides against the stop.
A holding tool which supports a wafer without touching it has also been proposed in DE-OS 26 09 754. The device works on the aerodynamic principle outlined above. The wafer is suspended on a gas cushion formed by gas flowing between the wafer and a holding plate and at the same time is held securely by the pressure effect of the gas moving preferably, in laminar flow, past the edge of the wafer. Damage caused to the wafer by the holding tool is naturally prevented.
The present invention is an apparatus that combines support of the wafer with only minor contact with the ability to securely hold the wafer on the holding tool.